[Elecraft] 33.25ft vertical with KXAT1 - Why did it work?

[Stephen W. Kercel]

Martin:

Antennas work by witchcraft and magic.  NEC is about as good a modeling 
program as you will find, but all antenna calculations are only a gross 
indication of expected performance. Actually, the problem is that the near 
field has extreme sensitivity to boundary conditions; these are impossible 
to model realistically, but they do affect performance measures such as 
feedpoint impedance. All antenna analyses are overly simplistic compared to 
the real antenna that they represent. Thus, comparing NEC studies to 
observed results often gives the appearance of witchcraft and magic.

Three things might account for your result. First, the idealized 
description of the antenna in NEC tends to depart from reality when you are 
trying to look at impedance for a feedpoint at the voltage loop. (Technical 
detail: Near the current loop, where dipoles are usually fed, the voltage 
along the antenna varies slowly with displacement. Thus, NEC's assumption 
that the voltage is uniform along a segment is approximately correct, and 
estimates of impedance are pretty close. Near the voltage loop, where 
you're feeding your antenna, the voltage along the antenna varies quickly 
with displacement. Thus, NEC's assumption that the voltage is uniform along 
the segment is far from correct, and estimates of impedance can be way 
off.  ) In my experience, the NEC estimate of impedance for voltage loop 
feedpoints is often overstated by a factor of 2 to 3. Second, your 
counterpoise is very near the ground, and the description of ground in 
EZNEC is (unavoidably) very crude. Third, you did not say how long your 
feedline is. A lossy feedline will have a non-trivially lower SWR at 
transmitter end than at the antenna end.

If you're getting 2000 km/watt in a K=8 storm, you've got a good 
antenna/feedline system.

Congratulations and 73,

Steve
AA4AK


At 05:27 PM 9/12/2005 +0100, Martin Gillen wrote:
>Hi.
>
>I was trying to build a field portable quick to
>install
>antenna and I really liked what I read about an end
>fed
>halfwave having a very low angle of radiation and only
>
>requiring a very short counterpoise however
>I remembered that the KXAT1 manual stated not to use
>lengths near a halfwave as they would be outside the
>matching range.
>
>So... I decided that I would cut an antenna to 33ft
>(20m halfwave), and then I would cut it back a foot at
>a time until the KXAT1 found a match.
>
>I started at 33.25ft vertical but I did leave a pair
>of
>18.5ft counterpoise wires running at about 2 feet
>above the ground connected to the ground terminal.
>
>I tuned up and ... 1.2:1 match on 20m!
>
>I then proceeded to work KF6GC with 2W (almost 4000km)
>in the middle of a K Index = 8 solar storm!  So I know
>the antenna worked well.
>
>I modelled it in EZNEC and the impedance at the base
>of that antenna should have been 970 + j 1700 ohms,
>so I am surprised that the KXAT1 was able to match it.
>
>Any comments?  Was it because I left the counterpoises
>connected that it worked so well?  If I remove the
>counterpoises in the NEC model the impedance increases
>and becomes inductive to 1456 - j 1819 ohms.
>
>Comments?
>
>PS.  I am only learning in this stuff so forgive any
>overly simplistic analysis!!
>
>Thanks,
>Martin.
>VA3SIE.
>
>
>
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